Color-Changing Badge Detects Blast Exposure

Crystal method: This nanopatterned crystal is designed to break at a specific supersonic blast frequency, changing the visible color of the material. The intact structure can be seen in the upper left corner, and the shattered structure in the lower right.

Over the last 10 years, thousands of troops have returned from Iraq and Afghanistan with traumatic brain injuries triggered by blasts from improvised explosive devices. Growing evidence suggests that the shockwaves produced by these explosions lead to injuries that are different from concussions suffered in car accidents and football games—and that even seemingly minor blasts, from which a soldier might walk away apparently unharmed, could damage the brain, especially with repeated exposure.

A new device being developed by researchers at the University of Pennsylvania School of Medicine could provide a simple way to measure the magnitude of explosions to which a soldier is exposed over time. It could also help scientists better understand the threshold for brain injury.

“Soldiers [with mild traumatic brain injury] can often appear normal, so it’s critically important to have some kind of objective measure to denote which soldiers have been exposed to a blast that is powerful enough to cause brain injury,” says Kacy Cullen, assistant professor of neurosurgery at Penn and leader of the study. “These devices wouldn’t diagnose brain injury, but they would indicate who needs a more thorough workup, and could influence decisions about when a soldier can return to action.”

The powerful blasts triggered by improvised explosive devices generate a supersonic wave followed by another shock wave called an overpressure wave. These forces are often strong enough to throw someone in the air, triggering the kind of blunt impact one might experience in a car accident. But many scientists believe that the waves themselves, in addition to the impact, can damage the brain.

The military has amped up efforts to measure the specific properties of explosions using helmet-mounted accelerometers and pressure sensors, but these devices have drawbacks. “They can be expensive, cumbersome, and require power to operate,” says Cullen. “Ours is a materials-based indicator, so you don’t need an internal power supply; the power from the blast induces the color change.”

As with a butterfly’s wing, the color of the material used in the detector is determined by its structure rather than chemical composition or pigment. It contains photonic crystals made up of layers of pores separated by columns a few hundred nanometers in width—the size of the pores and columns and how they are arranged within the structure determines the color of the sensor. When exposed to a shockwave, the columns collapse, either changing the color of the material or making it lose color altogether.